Automatic document feeder having a center drive mechanism for loading and unloading a document without skew

ABSTRACT

An automatic document feeder mounted removably to a computer input device having a medium receiving and supporting surface includes a plurality of interconnectable modular assemblies for defining a medium path that extends from a medium input tray to a medium output tray via the medium receiving and supporting surface of the computer input device. One of the modular assemblies is a main chassis assembly includes an integrally formed main chassis shell having a substantially planar base. The input tray has a document receiving surface with a concave shape in both the vertical and horizontal planes to help facilitate proper sheet stiffening and alignment for sheet loading purposes onto the receiving and supporting surface of the computer input device. A pair of spaced apart drive rollers mounted to a spring loaded drive axle engage in seriatim individual sheets for loading and unloading them onto the supporting surface of the computer input device. A motor driven transmission system is mounted to a central portion of the drive axle to balance the drive force applied by the drive rollers so that the individual medium sheets are loaded and unloaded without any substantial sheet skewing.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application of co-pending utilityapplication Ser. No.: 09/041,846, entitled "Document Backing LiftMechanism For Automatic Document Feeder," and its other co-pendingdivisional application Ser. No.: 09/181,573, entitled "AutomaticDocument Feeder Having An Input Tray Paper Stop And Pick Mechanism," andis further related to co-pending utility patent application Ser. No.:09/041,844, entitled "Modular Automatic Document Feeder For A Flat BedInput Device," and its associated co-pending divisional applicationsSer. No.: 09/181,571, entitled "Automatic Document Seeder HavingUniversal Output Tray," and Ser. No.: 09/181,572, entitled "AutomaticDocument Feeder Quick Release Hinge Assembly."

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to hard copy document apparatusand, more particularly, to cut sheet print media automatic documentfeeders.

2. Description of Related Art

Automated business machines for producing or reproducing hard copydocuments, such as copiers, printers, telecommunications facsimilemachines, document scanners, and the like, are well known commercially.Ideally, when working with cut sheet print media, a copying apparatusoften includes an automatic document feeder ("ADF") mechanism forautomatically loading and unloading single sheet sequentially to afunctional station where the copying apparatus performs anoperation--e.g., sequentially scans the fed document sheets for copying,faxing, displaying on a computer monitor, or the like. Following theoperation, the ADF then off-loads that sheet and feeds the immediatelyfollowing sheet of the document to the functional station. A sequentialflow of sheets by the ADF and positioning without the necessity ofmanual handling reduces the time required to accomplish the completefunctional operation. Media which already contains printed matter(hereinafter referred to generically as a "document") presents a needfor precise alignment to a scanning station in order to generate a truecopy.

Two major problems associated with business machines that include an ADFare the occasional mis-feed, commonly known as a "paper jam," and amultiple sheet feed. Paper jams interrupt operation and require manualcorrection of the problem before the automated conveying can berestarted. Multiple sheet feed can result in a missing page in the copy.

Typical commercial ADFs generally require a document sheet pre-pickingmechanism to ensure a single sheet gets to a pick roller, especially insystems having a horizontal input paper tray. Most commercial ADFs forscanners scroll the document page-by-page passed a stationary scan headand into an output tray. However, this makes pre-scanning and othermultiple scanning operations difficult or even impossible, leading todegraded scan output quality.

Belt type document feeders have been adapted to place a document onto aflat, transparent, scanning bed. One such system is shown in U.S. Pat.No. 5,342,133 (Canfield), assigned to the common assignee of the presentinvention.

A beltless drive system is demonstrated in allowed U.S. patentapplication Ser. No. 08/651,066 (Hendrix), assigned to the commonassignee of the present invention.

A third common problem is the inadvertent skewing of the document pagein the loading and unloading of paper sheets on the glass of a scanneror copier. Skewing can often lead to a paper jam. A first prior artmethod is to affix a paper drive mechanism axle to the ADF structure andturn the axle with a motor and gear train or timing belt linkage. Themotor and the linkage are also rigidly mounted to the ADF structure. Thedisadvantage of this method is that it is difficult to balance thenormal force between the two drive rollers. The stiff elastomerdurometer acts as a spring constant and any minor difference in theroller diameter, roller run-out, or roller position relative to the bedwill result in a significant difference in the normal force between thetwo rollers. A second prior art method is to spring load an axle to theADF structure, turning the end of the axle with a motor and gear trainor timing belt linkage. The motor is rigidly mounted to the ADFstructure. The linkage moves with the axle and pivots about the motorshaft. The disadvantage is that it becomes substantially impossible tobalance the normal force between the two drive rollers. The linkagegenerates unequal force when moving the drive roller axle in a forwarddirection versus a reverse direction. Because the linkage is positionedat the end of the axle, this unequal force teeters the drive roller axleand results in uneven normal forces. A third prior art method is toagain spring load the axle to the ADF structure, again turning the endof the axle with a motor and gear train or timing belt linkage with themotor and linage moving with the axle. Disadvantages are a difficulty inbalancing the normal force between the two drive rollers and thesusceptibility of a motor suspension to transportation shocks (the motorweight far exceeds the required drive roller normal force and thereforerequires a complicated suspension to lift the motor on one end whilemaintaining a balance force to the other end of the axle. A fourth priorart method is to spring load an axle to the ADF structure and turn theaxle in the middle. Both the motor and linkage move with the axle. Thedisadvantages of this method are that the motor weight far exceeds therequired drive roller normal force, and therefore requires a complicatedsuspension system in the middle of the ADF structure where it isgeometrically constrained and that the motor suspension is susceptibleto transportation shocks.

In order to have an ADF with a small workplace form factor (alsosometimes referred to as a "desktop footprint"), it is desirable toinput and output document sheets from vertically oriented trays. Onevertically aligned paper sheet input mechanism is taught by Hock et al.in related U.S. Pat. Nos. 5,320,436 and 5,326,090, assigned to thecommon assignee of the present invention.

There is a need for an inexpensive ADF, having a simple paper path andbeing adaptable for hard copy apparatus that employ a flat bed documentscanners.

SUMMARY OF THE INVENTION

In its basic aspects, the present invention provides a modular automaticdocument feeder "ADF" for use with a flat bed input device, such as aflat bed scanner or flat bed copier. The ADF has a set of three unitarychassis including a main chassis disposed between an upper chassis and alower chassis for defining a complete paper path onto and off of theflat bed scanner when the ADF is releasably mounted thereto. The modularconstruction of the three separate chassis facilitates ease of assemblyin a fast and efficient manner. Accordingly, it is an advantage of thepresent invention that provides an automatic document feeder having asimple, three chassis modular construction, creating a complete paperpath.

It is an advantage of the present invention that it permits simpleplastic injection molding manufacture of the chassis constructs.

It is an advantage of the present invention that the use of a minimalnumber of foundational ADF units minimizes the tolerance accumulationsamong features.

It is an advantage of the present invention that it has an attachmenthinge mechanism providing precision alignment of the ADF to the scannerwithout the use of tools.

It is an advantage of the present invention that it provides a quickconnect-disconnect of an ADF to a base unit.

It is another advantage of the present invention that it allows liftingof the ADF and direct access to the base unit copying surface to permithand copying such as for brochures and books.

It is another advantage of the present invention that it has a lowprofile, small footprint, document output tray.

It is a further advantage of the present invention that the use of aminimal number of foundational ADF units minimizes the manufacturingassembly time and skill requirements.

It is a further advantage of the present invention that it achievesmanufacturing cost reduction while providing a reliable ADF.

It is yet another advantage of the present invention that it eliminatesthe need for a pre-picking mechanism to ensure paper gets to a pickroller.

It is still another advantage of the present invention that it providesa paper drive mechanism having balanced drive roller normal forces toensure loading and unloading paper sheets without skew.

Other objects, features and advantages of the present invention willbecome apparent upon consideration of the following explanation and theaccompanying drawings, in which like reference designations representlike features throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned features of the preferred embodiments of the presentinvention and the manner of attaining them will become apparent, and theinvention itself will be best understood by reference to the followingdescription of the embodiments of the invention in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a perspective view of an ADF, which is constructed inaccordance with the present invention;

FIG. 2 is a rear, left, overhead perspective view of the ADF as shown inFIG. 1;

FIG. 3 is a rear, right, overhead perspective view of the ADF as shownin FIG. 1;

FIG. 4 is a front perspective view of a main chassis of the ADF as shownin FIG. 1;

FIG. 5 is a rear perspective view of the main chassis as shown in FIG.4;

FIG. 6A is a front perspective view of an upper chassis of the ADF asshown in FIG. 1;

FIG. 7 is a rear perspective view of the upper chassis as shown in FIG.6;

FIG. 8A is an exploded, front perspective view of a lower chassis of theADF as shown in FIG. 1, including an ADF attachment hinge in accordancewith the present invention;

FIG. 8B is front perspective view of the lower chassis of FIG. 8A;

FIG. 8C is a fragmentary cross-sectional view of the lower chassis ofFIG. 8A, taken along line B--B;

FIG. 8D is a front perspective view of an alternative embodiment of thelower chassis of FIG. 8A having a paper depressor element;

FIG. 8E is a perspective view of a crossbeam of the attachment hinge asshown in FIG. 8A;

FIGS. 8F, 8G, and 8H are detail perspective views of retainer hooks forthe attachment hinge as shown in FIG. 8A;

FIG. 8I is a front, right, low perspective view of the lower chassis ofFIGS. 8A-8C with the attachment hinge as shown in FIGS. 8A, 8B, 8E, 8F,8G, 8H mounted thereon;

FIG. 8J is a reverse perspective of detail of FIG. 8I;

FIG. 8K is a right, rear overhead perspective of the lower chassis withattachment hinge mounted as shown in FIG. 8I;

FIG. 8L is an enlarged fragmentary perspective view of the hingeattachment of FIG. 8K;

FIG. 8M is a rear perspective view of the lower chassis of FIG. 8A;

FIGS. 9A-9G are schematic drawings showing a document sheet loading andunloading sequence.

FIGS. 10A-10B are exploded, perspective drawings of the ADF as shown inFIG. 1 with a top case;

FIG. 11 is a perspective drawing depicting the orientation of the ADFrelative to a flat bed input device bezel;

FIG. 12 is a perspective detail drawing depicting an input tray paperstop and pick mechanism extracted from the present invention as shown inFIG. 4;

FIG. 13 is a perspective view showing the paper stop and pick mechanismof FIG. 12 in situ in a home position;

FIG. 14 is a perspective detail drawing showing the paper stop and pickmechanism of FIG. 12 in situ in a paper picking position;

FIG. 15 is a rear, left, perspective drawing of the main classis asshown in FIGS. 4 and 5 showing the mounting of the paper stop and pickmechanism of FIGS. 12-14;.

FIG. 16 is a perspective overhead view of a document sheet backingmechanism in accordance with the present invention as shown in FIG. 10;

FIG. 17 is a schematic elevation side view of the document sheet backingmechanism of FIG. 16 in the document sheet loading operationalcondition;

FIG. 18 is a schematic elevation side view of the document sheet backingmechanism of FIG. 16 in the document scanning operational condition;

FIG. 19 is a schematic elevation side view of the document sheet backingmechanism of FIG. 16 in the document sheet unloading operationalcondition;

FIG. 20 is a perspective drawing of the paper drive mechanism inaccordance with the present invention as shown in FIG. 10; and

FIGS. 21-25 illustrate the mechanical operation of the liftingmechanism.

The drawings referred to in this specification should be understood asnot being drawn to scale except if specifically noted.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is made now in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventors for practicing the invention. Alternative embodiments are alsobriefly described as applicable. Subtitles are provided herein forreference only; no limitation on the scope of the invention is intendednor should any be implied therefrom. Paper pick and feed drive softwareand firmware algorithms are well known in the art and beyond the scopeof the present invention; therefore, knowledge of a person skilled inthe art is presumed and no detailed description of such is providedherein nor is such necessary for an understanding of the presentinvention.

For ease in understanding the preferred embodiment of the presentinvention the following outline is provided:

A. Overall Modular Construction

B. Main Chassis

C. Upper Chassis

D. Lower Chassis

E. Attachment Hinge

F. Input Tray Paper Stop and Pick

G. Paper Drive/Document Backing

H. Paper Path

I. Lift Cam Mechanism

A. Overall Modular Construction

Referring now to the drawings and more particularly to FIGS. 1-3, 10 and11, there is shown an automatic document feeder 101 which is constrictedin accordance with the preferred embodiment of the present invention andwhich is adapted for use with a flat bed scanner 101. The automaticdocument feeder 101, will be referred to hereinafter as "the ADF" forsimplicity purposes.

Although in the preferred embodiment of the present invention referenceis made to a flat bed scanner 1101, those skilled in the art willunderstand any type of flat bed input device can be utilized, such as aflat bed copier.

The ADF 101 is modular in nature and generally comprises a set ofassemblies or chassis including a main chassis 401 (FIGS. 4-5), an upperchassis 601 (FIGS. 6A-7), and a lower chassis 801 (FIGS. 8A-8D). Thethree chassis 401, 601, and 801 when assembled together form a mediumpath (P) that extends from a medium input tray 403 best seen in FIG. 11,to a medium output tray 803 as illustrated in FIG. 8C. In understandingthe preferred embodiment of the present invention, it will be helpfulthroughout the description that follows, to refer generally to FIGS. 10and 1l. FIG. 10 shows an exploded view of the ADF 101 accompanied by anexemplary top case or outer shell 1001, while FIG. 11 illustrates theassembled ADF 101 with the top case 1001 in place relative to a genericscanner bezel 1101.

The three chassis 401, 601, and 801 are each unitary, injection moldedplastic parts adapted for simple assembly. Commercial embodimentdrawings are provided herein; therefore, it will be recognized by thoseskilled in the art that many features shown in the drawings are relatedto the molding process rather than to operational features. Injectionmolding process features are known to be design expedient andimplementation specific; therefore, no further explanation is requiredin order to understand the invention as claimed.

As can be recognized from FIG. 10B, when the ADF 101 is assembled withthe top case 1001 in place, the construct presents to the end-user asimple input document slot 1013, backed by an input tray 403, where thestacked document sheets are loaded laterally, print side down, i.e.,rearwardly facing. Note that the document is loaded in a "landscape"orientation regardless of the print orientation on the individualsheets. The only other feature of the assembled ADF construct apparentto the end-user is an output tray 803, protruding upwardly behind theinput tray 403, where collated output document pages are received fromthe internal paper transport mechanism following scanning or copying,for example. Output sheets are received within a gap between the inputtray 403 backside and the output tray 803 front side, best seen in FIGS.2 and 3. Any control panel features which may be provided in a specificimplementation, e.g., START and STOP pushbuttons, can also be providedin an accessible location on the top case 1001. This modularconstruction establishes the complete input-to-output paper path.

B. Main Chassis

Considering now the main chassis 401 in greater detail with reference toFIGS. 4-5, the main chassis 401 to the extent possible for a specificimplementation, is a unitary injection molded plastic part. The mainchassis 401 is provided with all the features to house the operationallyfunctional features of the ADF 101, namely motors, gear trains, shafts,paper picks and rollers, document lift assembly, and electroniccontroller equipment, and to establish an input paper path. Somefunctional features of the main chassis 401 are related to the interfitof the three chassis and paper drive mechanism and, therefore, furtherexplanation of these features is relevant.

As best seen in FIGS. 4-5, the main chassis 401 includes an integrallyformed main chassis shell 405 having a substantially planar base 407. Aninput tray 403 rises upwardly and rearwardly from the base 407. Theinput tray 403 has a document receiving surface 409 that has a concaveshape, in both vertical and horizontal planes. This double-concave shapeassists in both properly aligning the sheets of a multi-sheet inputdocument (not shown) and stiffening the document to facilitate sheetfeeding without jamming. See also, U.S. patent application Ser. No.08/706,032, filed by Hong on Aug. 30, 1996, for an Image-Related DeviceHaving Image-Medium Receiving Tray, and a Tray for Same, and a Methodfor Designing Such Tray, assigned to the common assignee of the presentinvention. More particularly, by permitting the document to conform tothe concave surface shape of the input tray 403 and having the sheetaligned in the input tray 403 along its longitudinal edge rather than inthe more conventional top or bottom edge first, causes each sheet tostiffen and thus facilitates a clean individual sheet pick. See also,U.S. Pat. No. 5,320,436, supra, col. 15, starting at line 65. A leftinput tray stanchion 411 and a right input tray stanchion 412, eachprotruding forwardly from the document receiving surface 409, furtherassist in aligning the sheets of the input document. As shown only in.FIGS. 10A-10B, a sliding paper length adjuster, or simply "slider," 1005can be mounted to the input tray 403.

As will be explained hereinafter in greater detail, the main chassisshell 405 includes three cavity areas 413, 417, and 419 respectivelythat are functionally related. The cavity area 413, is a central cavitythat is disposed between the cavity areas 417 and 419, which arehereinafter called a left side main chassis cavity 417, and a right sidemain chassis cavity 419.

In order to facilitate the specific implementation design expedients,the construct of the shell 405 includes a group of upper right sideinterfit features 415 and a group of left side interfit features 416.The right group of interfit features 415 is disposed on the right sideof the main chassis central cavity area 413, while the left group ofinterfit features 416 is disposed on the left side of the main chassiscentral cavity area 413. The configuration of the groups of interfitfeatures 415 and 416 facilitates the construct of the main chassis 401in a fast and efficient manner. In this regard, as best seen in FIGS.10A-10B, the upper chassis 601 mates between the interfit features 415and 416 permitting the upper chassis 601 to easily drop into the centralcavity area 413 of the main chassis 401. Moreover, the rear side of themain chassis 401 is also configured to matingly engage the lower chassis801. Thus, the lower chassis 801 is easily mounted on the rear side ofthe main chassis 401 by a set of four screws such as screw 1010. In thisregard, as best seen in FIG. 5, a set of molded screw retainer bushings,such as a bushing 50, are provided to align where the lower chassis 801is screw mounted to the main chassis 401.

When the upper chassis 601 is set in place relative to the main chassis401, a sheet feed gap is created between the input tray surface 409 anda rear portion of the upper chassis 601 at about the central cavity area413. Document sheets loaded into the input tray 403 drop sequentially inseriatim through this gap to travel onto the supporting surface of theflat bed input device 1101 as will be explained hereinafter in greaterdetail. Similarly, when the lower chassis 801 is mounted in placerelative to the main chassis 401, a sheet receiving gap is createdbetween the lower chassis 801 and the lower rear of the main chassis401.

Considering now the left side main chassis cavity 417 in greater detailwith reference to FIG.4, the cavity area 417 is sufficiently large involume to receive therein a left side stepper motor 103. The steppermotor 103 is mounted within the cavity area 413 by and suitable mountingdevices, such as the mounting devices 1010. The cavity area 417 is openoutwardly from the central cavity area 413 in order to facilitate easeof assembling the step motor 103 within the cavity 417.

Considering now the right side main chassis cavity 419 in greater detailwith reference to FIG. 5, the right side cavity area 419 also has asufficiently large volume to receive therein a right side stepper motor301, that is substantially similar in construction to the left sidestepper motor 103. The right side stepper motor 301, like the left sidestepper motor 103 is mounted within the cavity area 419 by conventionalmounting devices 1010.

As will be explained hereinafter with reference to FIGS. 10A-10B andFIG. 12, other mechanical elements of the ADF 101, such as a gear trainor transmission arrangement 1009, a paper pick 1201 and a paper driveshaft 1203, are coupled to the respective stepper motors 103 and 301 ina conventional manner which is well known to those skilled in the art.It is sufficient to note however, that the stepper motors 103 and 301are sequenced in operation by a controller 1003. The controller 1003, asbest seen in FIG. 10B is configured in the form of a printed circuitboard having electrical cabling indicated generally at 1007 thatinterconnects electrically the controller 1003 with the motors 103 and301 as well as other electrical components in the ADF 101. The printedcircuit board is dimensioned to be received within a front cavity area421 of the main chassis 401.

C. Upper Chassis

Considering now the upper chassis 601 in greater detail with referenceto FIGS. 6A and 7, a main cross beam 603 is positioned to traverse themain chassis first cavity 413 parallel to the input tray 403 (FIGS. 1-3and 10). A left side cantilever 607 and a right side cantilever 608extend forwardly from each end of the cross beam 603. Each cantilever607, 608 is provided with interfit features along the lower and sideedges thereof, or as otherwise necessary for a particularimplementation, which mate with the main chassis interfit features 416,415, respectively. Again, it is important to note that the upper chassis601 is a unitary injection molded plastic piece part; molding featuresand interfit features will be specific to a particular designimplementation. The upper chassis 601 is also secured to the mainchassis 401 with conventional fasteners 1010 as shown in FIGS. 10A-10B.

As shown in FIG. 7, the rear of the cross beam 603 is provided with aplurality of protruding fins 605. When the upper chassis 601 is mountedinto the main chassis 401, the fins 605 are in close proximity to theinput tray surface 409 as seen in FIG. 3. When a document is loaded intothe input tray 403, the document sheets are held between the input traysurface 409 and the upper chassis fins 605.

Molded leaf spring features 609, 610 are provided to retain a transfershaft 2003 (FIG. 20) using bushings 625, 626 as shown in FIGS. 7A-7C. Aseparator shaft 628 (FIG. 9A) is mounted to the upper chassis 601 usingmolded capture bearings and a bushing (not shown) and another moldedleaf spring 622 (FIG. 7) provided to retain a bushing (not shown). Theseparator shaft 628 bears a fixedly mounted central paper separatorwheel 631 (FIG. 9A). One end of the separator shaft 628 is coupled to amotor (not shown in this FIG.) via gearing (FIG. 2) and is driven duringa paper feed cycle.

A spring retainer 611 (FIG 6A) holds a compression spring (not shown).As shown in FIG. 10A, a tray sensor 701 and a pick sensor 703 as wouldbe known in the art.

D. Lower Chassis--Output Tray

The lower chassis 801 is shown in further detail in FIGS. 8A-8L, alongwith a quick release attachment hinge 821. The entire lower chassis 801is a unitary injection molded plastic part. Like the input tray 403, andas best seen in FIG. 8C, a document output tray 803 has a surface 805that provides a shallow concave curvature in a short height formfactor--in the dimensional region of half the paper width, e.g.,approximately one-half that of an A-size to legal size paper sheet equalto about 4.25 inches. Again this takes advantage of paper stiffnessinduced when it is shaped into a semi-cylindrical form along itslongitudinal axis. Thus, this embodiment can have a lower height formfactor that no longer needs to support a remaining half of a documentsheet by requiring such well known prior art mechanisms as output traypull-out extensions, wire frame clips, or the like. Returning to FIGS.8A and 8B, an upper chassis left stanchion 807 and an upper chassisright stanchion 808 are integrally molded parts of the lower chassis801, protruding forwardly from the output tray surface 805. As shown inFIG. 10A, the two stanchions 807, 808 are used for mounting the lowerchassis 801 via appropriately molded interfit features to the mainchassis 401 with fasteners 1010 any suitable known or proprietary type.

Along the lower edge of the output tray 803, a document sheet outputguide and shelf 809 extends forwardly. Output tray paper guide fins 811protrude upwardly from the shelf 809 to lift the leading lateral edge ofa sheet of output paper, directing it to follow along the output traysurface 805.

As best seen in FIGS. 2 and 3, the output tray 803, like the input tray403, has a substantially vertical orientation. The gap between the twotrays 403, 803 provides access for retrieval of the collated outputdocument sheets.

In an alternative embodiment as depicted in FIG. 8D, a paper depressor813 is added to the central region of the surface 805 of the output tray803. To mount the paper depressor, lower extremities 815 are formed tosnap-fit to the paper pick and stop shaft 1203 (as shown in FIG. 15).The paper depressor 813 is made of a low friction material such ascommercially available Cycoloy™, made by the General Electric company ofOne Plastics Avenue, Pittsfield, Mass. 01201. This design has beenproven to work reliably for plain papers from 16-pound to 24-pound typesat any acute angle less than forty-fives degrees to vertical as long asthe output tray height is at least one-half the document sheet length.With the paper depressor 813, the paper sheets are forced into acylindrical shape and stiffened. Without the depressor 813, the outputtray angle to vertical should be reclining sufficiently to retain thepaper sheets upright.

E. Attachment Hinge

Considering now the quick release attachment hinge 821 in greater detailwith reference to FIG. 8A, the hinge 821 enables the modular chassis401, 601, and 801 when assembled to be mounted to the flat bed scanner1101 with a precisely aligned document feed path relative to thesupporting surface of the scanner 1101. The hinge 821 facilitates atleast two different ADF/scanner orientations for document scanningpurposes coupled with a quick release feature for disconnecting the ADF101 from the scanner 1101. In this regard, as shown in FIG. 8A, thelower chassis 801 serves as a mount for the ADF hinge 821. As shown inFIG. 11, this attachment hinge 821 provides a dual purpose mechanism,both for mounting the assembled three chassis ADF 101 to either acompatible copier or scanner or to a bezel 1101 adapted to overlay sucha copier or scanner flat bed scanning surface and for raising andlowering the ADF off a flat bed bezel without removal of the entire ADFsystem.

Returning to FIGS. 8A and 8E-8H, a crossbeam 823 is provided with a setof hinge axle retainers 825, 826, 827, 828, 829, 830. Axle retainers825, 828, and 830 are rearwardly facing and axle retainers 826, 827, and829 are forwardly facing. As seen in FIGS. 8B and 8C, the lower chassis801 is provided with three axle casings 831, 832, and 833. Turning toFIG. 8A, three lower chassis attachment hinge axles 834 are provided toswivel mount the attachment hinge 821 to the lower chassis 801 in theshown manner via the axle retainers 825-830 and axle casings 831-833with each of the axle casings riding an axle between a complementarypair of reversed-facing axle retainers, e.g., the casing element 831 isbetween retainer elements 825 and 826; see also FIGS. 8I-8L.

As shown in FIGS. 8A and 8F-8H, a pair of attachment hinge flanges 835are mounted to the attachment hinge 821 by suitable fasteners 1010 anddepend from the crossbeam 823. As shown in FIG. 11, in combination withthe crossbeam 823, the flanges 835 are used to mount the assembled ADFto either a compatible copier or scanner or to a bezel 1101 adapted tooverlay such a copier or scanner flat bed scanning surface. The flanges835 are provided with dimensions sufficient not only to provide asteady, accurate aligning of the ADF, but also with a length sufficientto allow for lifting of the ADF vertically without removing the entireADF from the bezel 1101 or flat bed, such as when the document pages tobe scanned are in a book or magazine. To facilitate this function, thelower extremity of the hook 835 us provided with a flexible hook 837adapted to catch a complementary lip or edge of the bezel or scannerflange receiving slots (see e.g., FIG. 11, sleeve 1103).

As the ADF is swivel mounted to the hinge 821, ADF tilt-up stops areprovided to facilitate use of the ADF. FIG. 8B shows that the bottom ofthe lower chassis output document sheet guide and shelf 809 is providedwith depending ADF tilt-up stops 841. FIG. 8E shows that the crossbeam823 of the attachment hinge 821 is provided with rising protrusions thatform complementary, hinge tilt-up stops 842. The tilt-up stops 841, 842in the preferred embodiment are matched such that a 65-degree rotationupward of the ADF is permitted (see detail plane B--B, FIG. 8B). FIGS.8I-8L depict the interfit of the hinge features with the lower chassisfeatures. FIG. 11 shows the ADF 101 at its upwardly tilted positionrelative to the bezel 1101. Note that this also facilitates mounting theADF to a scanner provided with appropriate flange slots or a bezel 1101with receiving sleeves 1103.

F. Input Tray Paper Stop and Pick

Paper pick and transport mechanisms are also housed by the main chassis401. FIG. 12 shows an input tray paper stop and pick mechanism 1201 inaccordance with the present invention. FIG. 13 shows the paper stopmechanism 1201 mounted in position with respect to the input tray 403with the mechanism in a paper loading, or "home," position; the upperchassis is not shown in position in this drawing. FIG. 14 shows thepaper stop and pick mechanism 1201 mounted in position with respect toinput tray 403 with the paper stop and pick mechanism in a "retracted"position as it would be during a paper picking operation.

Centrally mounted on a paper pick and stop shaft 1203 is a paper pickroller 1205. The paper pick roller 1205, generally having a rubber orgrit wheel surface, is mounted via a fixed bearing hub 1207. A pair ofspaced apart spring loaded paper stop fence devices 1209 are mounted onopposite sides of the pick roller 1205. The stops 1209 protrude into thepaper path (P) and support from below a stack of sheets of paper (notshown) loaded into the ADF 101. As an individual sheet is picked fromthe stack by the pick roller 1205, the stops 1209 temporarily rotate outof the paper path allowing the picked single sheet to fall under theforce of gravity from the stack to start a journey along the paper pathto the output tray 801 as will be explained in greater detail. Once thepicked sheet has cleared the stops 1209, the stops 1209 snap back intothe their original resting positions to support from below any remainingsheets in the stack.

The stop fence devices 1209 are mounted on the shaft 1203 and furtherhelp to minimize teetering of the paper stack on the pick roller 1205.Considering now the paper stop device 1209 in greater detail withreference to FIG. 12, each paper stop fence device 1209 includes ahousing 1211, a one-way needle clutch 1213, and a biasing torsionalspring 1215, biasing the device toward its home position as depicted inFIG. 13. The housing 1211 has extended paper stop fence tabs 1217. Thetorsional spring 1215 provides enough force on the housing 1211 suchthat the tabs 1217 can support a number of sheets of a document (e.g.,50 sheets of legal size paper of up to 24-pound type). In operation,when a document is loaded (see arrows labeled "P") into the input tray,face down--i.e., with the print side facing input tray surface 409--thepaper sheets (not shown) have their side edges sitting on the top edgesof the paper stop fence tabs 1217 which are protruding substantiallyhorizontally through aligned apertures 1301 provided in the surface 409of the input tray 403. Similarly, an aperture 1303 is provided in theinput tray surface 409 for the paper contact surface 1206 of the pickroller 1205 to extend forwardly of the input tray 403.

A paper pick drive shaft 1203 connects the paper stop and pick mechanism1201 to a stepper motor 103. Turning briefly to FIG. 15, the mounting ofthe drive shaft 1203 to a motor 103 is made with standard bushings 1501.

As a sheet of paper is center-picked, the force of a moving documentsheet imparted by the pick roller 1205 is sufficient to release theclutch 1213 and push the tabs 1217 out of the way as shown in FIG. 14.The torsional spring 1215 causes the tabs 1217 to bounce back to thehome position after the trailing edge of the paper clears the outermostpoint on the tabs. In other words, by adding a paper stop and pickmechanism 1201, the present invention takes advantage of the verticalinput tray 403 in that paper falls into the pick roller by gravity,eliminating the need for pre-picking mechanism.

G. Paper Drive/Document Backing

A paper drive and document backing mechanism is mounted to theunder-carriage of the main chassis 401. FIG. 16 shows an assembled paperdrive 2001(detailed in FIG. 20) and a document backer 1601, referred toin the assembled combination as a paper pressure mechanism 1611.

The ADF of the present invention uses the technique of sequentiallyloading and unloading original document sheets onto the scanner flat bedglass 1701 (FIGS. 17-19) using a document backing method taught in U.S.patent application Ser. No. 08/651,066 (Hendrix et al.), supra. In orderto provide the highest quality scanned version, it is desirable to pressthe document sheets firmly against the glass 1701 of the scanner. Theassembly forming a document backer 1601 includes a lift plate 1603 is asubstantially rigid, planar material, such as a molded ABS plastic.Mounted on the lower surface of the lift plate 1603 is a foam rubber pad1605, preferably of polyurethane, commercially available from Boydcompany of 13885 Ramona Avenue, Chino, Calif., providing a documentbacking pressure pad for a document sheet on the scanning bed glass 1701at a scanning station. To the lower surface tension of the foam pad, athin plastic film 1607 preferably of polycarbonate film, trade name"Lexan", commercially available from General Electric company of OnePlastics Avenue, Pittsfield, Mass 01201, is provided to act as alow-friction contact with document sheets.

The combination of materials used in the document backer assembly 1601and the lifting of the assembly during loading and unloading of printmedia sheets from the flat bed of a scanner, or bezel, not only reducesthe friction, but also minimizes the electrostatic build-up, improvingreliability of sheet feeding.

In the preferred embodiment, the document backing assembly 1601 ismounted to the undercarriage of the main chassis 401 by slipping theassembly through the main chassis first cavity 413. The document backingassembly 1611 is inserted through the first cavity 41, of the mainchassis 401; front hooks 1610, 1612 mate with molded features of themain chassis under carriage toward the front thereof; spring 1614 loadedbushings 1613 are provided to hold the document backing assembly 1601 inmolded features of the main chassis toward the rear thereof, adjacentthe first cavity 413 as shown in FIG. 10A. The present invention solvesthe problem of getting a document sheet between the glass 1701 and theplastic film 1607 by lifting the assembled paper drive 2001 and documentbacker 1601 as the document sheet is loaded and unloaded from the glass.A lift cam mechanism 1609 moves between:

(1) a forward position as shown in FIG. 17, lifting the document backingassembly 1601 to permit a sheet of paper to enter between the assemblyand the scanner bed glass 1701 as indicated by arrow 1611,

(2) a central position as seen in FIG. 18, dropping the document backingassembly 1601 onto the scanner bed glass 1701, and

(3) a rearward position as shown in FIG. 19, lifting the documentbacking assembly 1601 to permit a sheet of paper to be driven off thescanner bed glass 1701 as indicated by arrow 1613.

The cam lift mechanism 1609 will be described hereinafter in greaterdetail.

FIG. 20 shows detail of the paper drive mechanism 2001. The reversiblestepper motor 301 is coupled to a transfer shaft 2003 via a conventionalgear train, or other transmission, 2005. A pair of paper drive rollers2007, 2008 are mounted on a paper drive roller axle 2009. The paperdrive roller axle 2009 has spring-loaded mountings 2011, 2012 to the ADFmain chassis 401 structure as depicted in FIG. 15. A gear train linkage2013 is used to turn the paper drive rollers 2007, 2008 via axle 2009 atthe approximate midpoint of the axle. The gear train linkage 2013 moveswith the axle and pivots about the transfer shaft 2003. Note that boththe motor 301 and the transfer shaft 2003 is rigidly mounted to the ADFmain chassis 401 structure. As a result of this configuration, thenormal force of the two drive rollers 2007, 2008 is balanced because thegear train linkage force is applied in the middle of the two driverollers and therefore is equally distributed. The balanced normal forceensures loading and unloading of sheets of the document without skew. Inshort then, the spring loaded drive axle 2009 coupled to the systemtransmission system activates the elastomeric drive roller 2007 and 2008to heir facilitate the loading and unloading of a document sheet ontothe supporting surface of the flat bed scanner 1101 without anysubstantial paper skew. The gear train linkage 2013 is used to providemotion to a lift cam during paper loading and unloading to hoist thedocument backer 1601 off the glass (FIGS. 17 and 19).

H. Paper Path

FIGS. 9A-9G depict the dynamics of the paper path established by themain chassis 401, upper chassis 601 and lower chassis 801. In thisembodiment, a bezel 902 is attached to the FIG. 9A shows the apparatus101 in the condition where a document sheet 901 is loaded. The paperstop and pick mechanism 1201 is in the position shown in FIG. 13, andthe document sheet 901 is resting on the two paper stop and pickmechanisms 1201 and paper pick roller 1205. The lift cam mechanism 1609is in the same position as shown in FIG. 18, with the document backingassembly 1601 in its lowered position.

FIG. 9B demonstrates pertinent apparatus feature positions during apaper pick operation. The paper pick roller 1205 is motor-activated,turning so as to pull the sheet 901 downward into the tray. The sheet901 is fed downward and shaped between the input tray surface 409 andthe upper chassis 601 fins 605. The tabs 1217 of the paper stop and pickmechanism 1201 are moved out of the way by the sheet 901 as describedwith respect to FIG. 12. The leading edge 903 of sheet 901 is directedtoward the drive rollers 2007, 2008. Activation of the drive rollers2007, 2008, turning in a rotation that will receive the leading edge 903of sheet 901 and direct it toward the scanning station, also moves thecam mechanism 1609 to its forward position, lifting the document backingassembly 1601 such that the sheet will slip underneath.

During a paper pick cycle, as shown in FIG. 9C, the leading edge 903 ofthe sheet 901 contacts the surface of the glass bed before reaching thedrive rollers 2007, 2008. It has been found that the buckling of thesheet as it impact the glass 1701 and before reaching the drive rollers2007, 2008 de-skews any offset of the sheet.

Turning to FIG. 9D, the drive rollers 2007, 2008 drive the sheet ontothe glass 1701. The trailing edge of sheet 901 remains in contact withthe drive rollers 2007, 2008 when the sheet is positioned for scanning.The lift cam mechanism 1609 returns to its center position and drops thedocument backing assembly 1601 onto the sheet 901.

Following scanning, as shown in FIG. 9F, the drive rollers 2007, 2008are driven in a reverse direction from a sheet feed cycle. The lift cammechanism 1609 is moved to its rearward position as shown in FIG. 19,lifting the document backing assembly 1601 to permit a sheet of paper901 to be driven off the glass 1701. The trailing edge of the sheet 901that had been retained under the drive roller 2007, 2008 has now becomethe exit "leading edge" 905 of the sheet. The lower chassis outputdocument sheet guide and shelf 809 receives the exit leading edge 905.The sheet is driven between the output tray drive rollers 1503 (see alsoFIG. 15). The output tray drive rollers 1503 directs the sheet 901upwardly into the output tray 803 where it is retained behind the shelf809 as shown in FIG. 9G. The lift cam mechanism 1609 is returned to itscenter position.

I. Lift Cam Mechanism

Referring now to the drawings and more particularly to FIGS. 9A-G and21-25 there is shown a lift cam mechanism 1609 that facilitates thelifting of an assembled paper drive 2001 and document backer 1601 duringthe loading and unloading of a document sheet onto the transparent glasssupporting surface 1701. In this regard, the lift cam mechanism 1609solves the problem of transporting such a document sheet between thesupporting surface 1701 and the plastic film 1607 during such loadingand unloading operations.

Considering now the lift cam mechanism 1609 in greater detail withreference to FIGS. 9A-G and 21-25, the lift cam mechanism 1609 generallyincludes a cam linkage indicated generally at 1615, and a differentialgear train arrangement indicated generally at 1617. The cam linkage 1615and the differential gear train arrangement 1617 in combinationfacilitate a passive automatic lifting action of the document backer1601 to substantially reduce friction and electrostatic build up forreliable sheet feeding purposes.

As best seen in FIGS. 21-25, the cam linkage 1617 gernerally includes anelongated transfer shaft 2003 that is supported for rotational movementbetween a motor plate 1619 and the chassis 401. A pair of transfergears, 8001 and 8002 respectively, are pressed mounted spaced from oneanother on the shaft 2003 to facilitate a lifting action enabled by afree spinning lift cam 8005 that will be described hereinafter ingreater detail,

The transfer gear 8001 has a larger diameter than the transfer gear 8002and is mounted relative to the shaft 2003 so that it engages a motorpinion gear 1621 extending from the drive motor 103. In this manner, themotor pinion gear 1621 translates the rotational drive of the motor 103to the transfer gear 8001 that in turn causes the shaft 2003 to berotated about it axis at a desired rotational speed.

The lift cam 8005 and a free spinning transfer idler gear 8003 aremounted inwardly of the transfer gear 8002 on the shaft 2003. The idlergear 8003 and the lift cam 8005 are slightly spaced from one anotherwith the lift cam 8005 being mounted furthest from the distal end of theshaft 2003. An elongated lift cam pin 8006 is mounted fixedly by wedgingto the lift cam 8005 and extends outwardly therefrom parallel to theshaft 2003. Mounted to the distal end of the lift cam pin 8006, is afree spinning lift cam composite gear 8004, having a small diameterportion and a large diameter portion. The small diameter portion engagesthe idler gear 8003 and the large diameter portion engages the transfergear 8002.

In order to move a document onto and off the scanner bed glasssupporting surface 1701, a free spinning drive roller 2007 is pressedmounted to a drive shaft 2009 having a press fitted drive gear 8008mounted to its distal end. The drive shaft 2009 is mounted spaced fromand parallel with the transfer shaft 2003 and is supported between themotor plate 1621 and a free spinning cluster housing 8010. The clusterhousing 8010 mounted inwardly of the drive gear 8008 and is supported onand between the transfer shaft 2003 and the drive shaft 2009. Thecluster housing 8010 has fixed thereto an elongated housing pin 8009that extends outwardly therefrom in a plane parallel with the shafts2003 and 2009. A free spinning cluster housing composite gear 8007 ismounted on the pin 8009 between shafts 2003 and 2009. The composite gear8007 includes a large diameter portion that engages the drive gear 8008and a small diameter portion that engages the transfer idler gear 8003to farther facilitate a lifting action as will be hereinafter described.

As best seen in FIGS. 24-25, the lift cam 8005 includes a pair of spacedapart stops 8005A and 8005B that limit the movement of the lift cam 8005as will be explained hereinafter in greater detail.

Considering now the operation of the lift cam mechanism 1609 withreference to FIGS. 9A-G and 21-23, the lift cam 8005 begins in a downposition as best seen in FIG. 21. As the motor pinion gear rotates in aclockwise direction in engagement with the transfer gear 8001, thetransfer gear 8001 imparts a counter clockwise rotation to the transfershaft 2003. The transfer shaft 2003, in turn, causes the lift cam 8005to lift and to move to a final lifted position as shown in FIG. 22.

More particularly, as the transfer gear 8002 is fixed to the shaft 2003,the gear 8002 rotates in the same counter clockwise direction and at thesame rotational speed as that of the shaft 2003. The idler gear 8003,however, remains in a fixed position due to the friction between thedrive roller 2007 and the scanner glass bed 1701. In this regard, theidler gear 8003 locks the gear train of gears 8003, 8007 and 8008. In alike manner as the gear train of gears 8003, 8008 and 8009 is fixed, thedrive shaft 2009, its associated drive roller 2007 and cluster housing8010 also remain fixed in their respective positions. However, the freespinning lift cam composite gear 8004, rides along the transfer gear8002 and the idler gear 8003, and rotates in response to cause the liftcam 8005 and lift cam pin 8006 to move in unison, pivoting in aclockwise direction relative to the transfer shaft 2003.

Once the lift cam 8005 has moved to its final lifted position asillustrated in FIG. 22, the drive roller 2007 starts driving thedocument forward. More particularly, the drive roller 2007 impartsmotion to the document in the following manner. Motion of the lift cam8005 is inhibited by the stops 8005A and 8005B that are wedged on thelift plate 1603. With the lift cam in a fixed position, the lift camcomposite gear 8004 continues to rotate on pin 8006 in a clockwisedirection that now in turn, imparts a rotational motion to the geartrain comprising gears 8003, 8007 and 8008. In this regard, the gear8003 is rotated in a counter clockwise direction, which motion iscoupled to the drive gear 8008 via the clockwise motion of gear 8007about pin 8009. From the foregoing, it should be understood that thedocument is now pulled forward under the frictional force of the driveroller 2007. The drive roller 2007 rotates in a counter clockwisedirection due to the cluster housing 8010 being held in a fixedposition, while the gear 8007 rotates about pin 8009 to impartrotational motion to drive gear 8008 and shaft 2009.

After the document has been moved into a proper position for scanning,the lift cam 8005 is moved to its original down position as illustratedin FIG. 21. In this regard, the transfer shaft 2003 starts to rotate inan opposite clockwise direction, that in turn, causes the transfer gear8003 to rotate in the same clockwise direction. The lift cam compositegear 8004 riding along gears 8002 and 8003 rotates in an oppositedirection causing the lift cam 8005 and its associated pin 8006 to pivoton the transfer shaft 2003. Motion of the drive roller 2007, drive shaft2009 and the cluster housing 8010 remain fixed as no rotation motion isimparted via gears 8007 and 8008. In this regard, it should beunderstood that as the motor pinion gear rotates in a counter clockwisedirection, it imparts a clockwise motion to the transfer shaft 2003 viathe transfer gear 8001. The rotation of the transfer shaft 2003 causesthe transfer gear 8002 to rotate in the same direction at the samerotational speed, while the idler gear 8003 remains in a stationaryposition. Again, the friction between the drive roller 2007 and thedocument locks the gear train comprising the gears 8008, 8007, and 8003.Thereafter, the lift cam 8005 remains in a fixed position while thedocument is scanned.

After the document has been scanned, the lift cam 8005 is lifted topermit the document to be moved off of the scanner glass bed 1701. Inthis regard, the motor pinion gear is rotated in a counter clockwisedirection that in turn imparts a clockwise rotation to the transfershaft 2003 and the transfer gear 8002. That is shaft 2003 rotates viathe engagement of the transfer gear 8001 with the motor pinion gear, andthe gear 8002 rotates as it is fixed to the shaft 2003. As explainedearlier, the friction between the drive roller 2007 and the documentlocks the gear train of gears 8003, 8007 and 8008 in a fixed position.

The lift cam 8005 then begins to lift. The composite gear 8004 freespins on pin 8006 and rides along gears 8002 and 8003 causing the liftcam 8005 and its fixed pin 8006 to pivot about the transfer shaft 2003.The drive roller 2007, and drive shaft 2009 however remain in a fixedposition. In this regard, the housing cluster 8010, and gears 8007, 8008remain fixed thus, no rotational motion is imparted to the drive shaft2009. The final lifted position of the lift cam 8005 is illustrated inFIG. 23, where the stop 8005A and 8005B inhibit further motion by thelift cam 8005.

Once the lift cam 8005 has been lifted to its final position asillustrated in FIG. 23, the document is now ejected out from itssupporting position on the scanner glass bed 1701. In this regard, whilethe transfer shaft continues to rotate in a clockwise direction, thelift cam composite gear 8004 rotates in a stationary position counterclockwise about pin 8006. The rotation of gear 8004 is imparted to gear8007, which in turn rotates the drive gear 8008, drive shaft 2009 anddrive roller 2007 in a clockwise direction. In this manner, theclockwise rotation of the drive roller 2007 moves the document off thescanner bed 1701 in a reverse direction.

After the document has been removed from the scanner bed 1701, thepinion gear rotational direction is reversed, which in turn, causes thetransfer shaft 2003 to rotate in a counter clockwise direction. Thecounter clockwise rotation of the shaft 2003 imparts motion to the liftcam 8005 to return it to a down position. More particularly, thecomposite gear 8004 rotates in a clockwise direction and rides alonggears 8002 and 8003 causing the lift cam 8005 and its associated pin8006 to pivot about the transfer shaft 2003. The friction between thedrive roller 2007 and the glass scanner bed 1701 locks the gear train8008, 8007, and 8003 in a fixed position. The lift cam 8005 comes torest in it down position read for the next document.

From the foregoing, it should be understood by those skilled in the artthat the lift cam 8005 is driven in a predetermined sequence that startswith the lift cam 8005 being position in a down position as illustratedin FIG. 21 and ending in the same down position. The sequence includes:

1. moving the lift cam 8005 upwardly from a down position to an extendedposition that permits the document to be moved onto the scanner bed1701;

2. driving the document onto the scanner bed 1701 via the drive roller2007;

3. lowering the lift cam 8005 to its down position prior initiating ascanning operation;

4. waiting for the document scanning operation to be completed;

5. moving the lift cam 8005 upwardly from the down position to theextended position to permit the document to be moved off of the scannerbed 1701. Waiting for a complete document scanning operation to beperformed;

6. driving the document in a reverse direction off the scanner bed 1701;and

7. lowering the lift cam 8005 to its down position to wait for anotherdrive/eject cycle.

The foregoing description of the preferred embodiment of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form or to exemplary embodiments disclosed.Obviously, many modifications and variations will be apparent topractitioners skilled in this art. For example, in lieu of injectionmolded plastic, the chassis are amenable to sheet metal stamping orother known fabrication techniques. Similarly, any process stepsdescribed might be interchangeable with other steps in order to achievethe same result. The embodiment was chosen and described in order tobest explain the principles of the invention and its best mode practicalapplication, thereby to enable others skilled in the art to understandthe invention for various embodiments and with various modifications asare suited to the particular use or implementation contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. A lift cam mechanism for a document backer, comprising:a plurality of interconnectable modular assemblies including a main chassis; a paper drive mechanism mounted in said main chassis for loading and unloading in seriatim individual ones of the medium sheets as they are loaded and unloaded onto and off of a receiving and supporting surface for a computer input device; a cam linkage mounted in said main chassis and coupled to the document backer for facilitating its lifting to permit a document sheet to pass thereunder onto and off said supporting surface in a substantially friction free manner without an electrostatic build up and for facilitating its lowering onto the document sheet prior to a scanning operation; and a differential gear train arrangement cooperates with said cam linkage to facilitate the sequencing of raising the document backer to a raised position, advancing said document sheet onto said supporting surface, lowering the document backer in an overlaying relationship onto said document sheet to facilitate document scanning, raising the document backer to a raised position, ejecting the document sheet from said supporting surface in a reliable sheet feeding manner, and lowering the document backer onto the supporting surface in a ready position in anticipation of covering another document sheet.
 2. An automatic document feeder for a flat bed computer input device having a planar sheet receiving surface, comprising:a planar base member removably mounted to the flat bed computer input device for aligning a document path with the planar sheet receiving surface; a paper drive apparatus mounted to said base member for sequential loading and unloading of a document sheet onto the planar sheet receiving surface; said paper drive that sequentially loads and unloads the document sheet onto the planar receiving surface without skew including:a drive shaft mounted to said base member; a pair of spaced apart drive rollers mounted on said drive shaft; a transfer shaft mounted to said base member, said transfer shaft is substantially parallel with said drive shaft; a gear train coupled to said transfer shaft that drives the transfer shaft at a desired speed and direction; another gear train linkage coupled between a substantially center portion of said transfer shaft and a substantially center portion of said drive shaft and substantially equal distance from said pair of drive rollers to distribute the drive force of said motor substantially equally between said rollers.
 3. An automatic document feeder for a computer input device having a medium receiving and scanning surface, comprising:a plurality of interconnectable chassis for defining a medium path of travel extending between a medium input tray and a medium output tray via the medium receiving and supporting surface of the computer input device; said plurality of modular assemblies including a main chassis having a paper drive mechanism mounted therein for loading and unloading in seriatim individual ones of the medium sheets as they are loaded and unloaded onto and off of the receiving and supporting surface of the computer input device; said paper drive mechanism for engaging in seriatim individual ones of the medium sheets as they are loaded and unloaded onto and off of the receiving and scanning surface of the computer input device to substantially eliminate sheet skewing during the loading and unloading process.
 4. An automatic document feeder according to claim 3, further comprising:a motor mounted to said main chassis having a gear train; a transfer shaft coupled to said gear train at one of its ends and mounted at the other one of its ends to said main chassis; a gear train linkage coupled to said transfer shaft; and a spring loaded paper drive axle coupled to said gear train linkage to facilitate the driving of said drive rollers by said motor and for supporting said drive rollers in their spaced apart relationship.
 5. An automatic document feeder according to claim 4, wherein said paper drive mechanism further includes a pair of space apart spring loaded mounting mounted to said main chassis for supporting said paper drive axle in proper orientation to permit the drive rollers to engage the individual sheets of print medium as they travel in seriatim along a portion of the medium path between the input tray and the output tray.
 6. An automatic document feeder according to claim 5, wherein said drive rollers are elastomeric drive rollers.
 7. An automatic document feeder according to claim 5, wherein said gear train linkage is coupled to said drive axle equally spaced from the drive rollers so that the gear train linkage force is applied equally to the drive rollers to substantially prevent sheet skewing during the loading and unloading process.
 8. An automatic document feeder according to claim 7, wherein said gear train linkage is a pivoting gear train linkage to further facilitate the loading and unloading process.
 9. A balanced drive mechanism for transporting a medium sheet to and from a receiving and scanning surface in a computer input device, comprising:a plurality of interconnectable modular assemblies for defining a medium path of travel extending between a medium input tray and a medium output tray via the medium receiving and scanning surface of the computer input device; a pair of spaced apart drive balanced drive rollers mounted in one of said plurality of interconnectable chassis for engaging in seriatim individual ones of the medium sheets as they are loaded and unloaded onto and off of the receiving and scanning surface of the computer input device to substantially eliminate sheet skewing during the loading and unloading process; a motor mounted in the one of said plurality of interconnectable chassis, said motor having a gear train; a transfer shaft coupled to said gear train at one of its ends and mounted at the other one of its ends to the one of said plurality of interconnectable chassis; a gear train linkage coupled to said transfer shaft; and a spring loaded paper drive axle coupled to said gear train linkage to facilitate the driving of said drive rollers by said motor and for supporting said drive rollers in their spaced apart relationship.
 10. A balanced drive mechanism according to claim 9, wherein one of said plurality of interconnectable chassis is a main chassis; andwherein said paper drive mechanism further includes a pair of spaced apart spring loaded mountings mounted to said main chassis for supporting said paper drive axle in proper orientation to permit the drive rollers to engage individual sheets of print medium as they travel in seriatim along a portion of the medium path between the input tray and the output tray.
 11. A drive mechanism according to claim 10, wherein said drive rollers are elastomeric drive rollers.
 12. A drive mechanism according to claim 10, wherein said gear train linkage is coupled to said drive axle equally spaced from the drive rollers so that the gear train linkage force is applied equally to the drive rollers to substantially prevent sheet skewing during the loading and unloading process.
 13. A drive mechanism according to claim 12, wherein said gear train linkage is a pivoting gear train linkage to further facilitate the loading and unloading process. 